In Cecil’s answer to the question “Why do singers use vibrato?”, he says that vibrato is added into a singer’s sound for effect, and that good singers can sing without it.
I study classical voice at the University of Washington, and Cecil is basically wrong. The fact of the matter is that the natural tendency of the vocal cords is to flap around quite a bit. While any singer can sing without vibrato, and many singers who have not had any or very much voice training might think that a straight tone is perfectly healthy, the truth is that singing without vibrato imparts a very small amount of unwanted tension into the voice, and when a singer gets into his or her higher register, a tone without vibrato often sounds downright horrible, and feels even worse.
As a singer climbs higher into his or her range, a greater amount of release is required. You will be very hard pressed to find an operatic tenor who can sing a high C without letting a lot of vibrato come in.